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been adopted, it's assigned to a server in the data center. That server, however, is rarely used at any more than 5% to 10% of its computing capacity, which results in the proliferation of power-hungry systems that aren't working up to their full potential.

Server virtualization dramatically changes this paradigm in that it allows numerous virtual machines (VMs) to coexist and run on the same physical servers. That can vastly reduce hardware needs and environmental requirements. Consolidating servers is easy and offers many benefits, but making the consolidation work correctly can be a challenge.

One immediate benefit of optimizing server consolidation is that you'll use fewer physical servers. That, in turn, reduces the data center's capital hardware costs and space requirements and lowers collateral costs of power and cooling systems.

Introduction to Virtualization e-book

This article is excerpted from Chapter 2 of the Introduction to Virtualization e-book, which covers the basics of server virtualization technology. Learn about server consolidation, disaster recovery, high availability and more.

There is often a compelling return on investment argument with these factors alone. However, since there are fewer hardware devices that could fail, maintenance costs can also be kept in check. Fewer physical servers also means fewer costly annual service contracts to pay. You can reallocate older servers displaced in a technology update to other less critical tasks, extending the hardware's useful life. And it's easier to standardize on management tools when there are fewer servers. These benefits -- together with the tools used for VM management -- can facilitate data center management. Consolidation can also mitigate the need to add staff as the data center grows.

Perhaps the most overlooked and often understated benefit of consolidation is enhanced flexibility. A VM is basically a file that can be moved and copied between servers and storage systems. An administrator can create and configure a new VM from a basic image in minutes. Similarly, you can migrate a working VM to a more powerful server or copy it to backup storage at a disaster recovery site -- usually without disrupting the machine or its users.

Hardware failure and overgrowth One of the most obvious risks with server consolidation is hardware failure. In a traditional environment, a server fault would affect the application and users running on that physical server. However, a server failure in a virtual environment can disable all VMs that reside on it.

While techniques like server clustering help maintain high availability (HA), that practice is usually reserved for mission-critical VMs and often leaves secondary VMs on a single server. Consequently, administrators must track VM distribution across the enterprise to ensure that important VMs are running on adequate HA hardware. It's equally important to arrange backup cycles that meet the recovery point objective and recovery time objective for each VM.

A proliferation of virtual machines will also affect server virtualization. Just as conventional data centers faced problems with server sprawl, the ease and speed of creating new VMs can precipitate similar conditions as administrators quickly deploy a new VM for each new application or task.

"It's often referred to as 'VM sprawl,' but it's usually just a symptom of other machine management problems within an organization," said Bob Plankers, a technology consultant and blogger for The Lone Sysadmin. He noted that adequate planning and management tools are important to mitigate sprawl.

VM sprawl can easily lead to software licensing violations and can further complicate maintenance, since there is no direct link between an application's VM and the underlying physical server. While this abstraction is a desirable feature of virtualization, it can have undesirable consequences for careless administrators.

A final issue that often goes unnoticed is VM assignment. Although a virtual machine should function on any suitable piece of hardware, it might not be prudent to combine certain VMs, depending on their resource demands.

"Server consolidation isn't a good solution for all applications," said Rand Morimoto, president of Convergent Computing, a solution provider in Oakland, Calif. Morimoto noted that demanding database applications can be problematic when virtualized and may cause performance problems when consolidated with other VMs on the same server.

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